This invention relates to improvements in heavy duty vehicles and in particular to improved means for providing accurate and structurally sound guidance of vehicle axles while permitting substantial vertical axle travel relative to the frame during articulation over rough terrain.
A common method of locating and guiding an automotive rear axle is by use of the so-called 4-link system. There are two horizontal longitudinal links (often called "torque rods") mounted below the axle, and two horizontal splayed upper ones which diverge away from one another at equal angles to the longitudinal axis of the frame. These upper rods not only complete the parallelogram with the lower ones, as viewed from the side, but by angle-mounting them in plan view they will resist lateral or transverse axle forces and movement at the level at which they are attached. (This level on the axle, always at the centre-line of the vehicle, is known as the "roll-centre", as it is the centre about which the body rocks or rolls relative to that axle.)
Heavy trucks commonly have frame widths between 30 and 34 inches. There is thus very limited lateral space in which to locate splayed rods. If the rods are to be kept to a reasonable length, if the degree of rotary movement at the bushings is to be kept within acceptable limits, and if the swing arcs of the rods are to be kept relatively flat, the included or splay angle .theta. as measured in a horizontal plane between the two upper rods must be reduced. This is fine geometrically but unfortunately it results in higher loads on the rods and their bushings and brackets for any given transverse axle load. This is based on a consideration of the triangle of forces. Thus, for the force triangle ABC, the force represented by side AC=transverse axle load or force, while for the other two sides AB=BC=rod load. If the splay angle between the rods .theta.=60.degree., the force triangle is equivalent and thus the 3 forces are equal. If .theta. is less than 60.degree., the rod load is greater than the transverse load.
If the splay angle .theta. were increased well beyond 60.degree. to, say, about 90.degree., it can readily be seen that the new shape of triangle drastically reduces the rod loads. However, the narrow truck frame width, as used on heavy trucks, restricts the rod length permissible with this relatively large splay angle. Also, the longitudinal or effective length of these short rods is only about 71% of their eye-to-eye length. The limitation imposed on the maximum swing arc length restricts the vertical travel possible by the axle.
Thus, it can be seen that with conventional 4-link systems incorporating a pair of splayed upper rods that in practice a compromise must be arrived at. While it is desirable to increase the splay angle to reduce the loadings referred to above, such increase in angle reduces the effective rod length together with the vertical axle travel.
Another type of system for locating an axle of a heavy duty vehicle utilizes one longitudinal torque rod and one transverse "Panhard" or track rod. The two rods are located at 90.degree. to each other in plane view. The longitudinal rod absorbs all the upper torque forces and transfers them to a crossmember in the frame. The transversely oriented panhard rod (having one end attached to a frame main rail and another to a bracket on the axle) swings in an arc, as viewed from the front or rear, preventing a symmetrical suspension. Due to the limited swing radius of the panhard rod there can be considerable lateral movement between the axle and the frame. Also, all the transverse forces are transferred via the panhard rod to the one frame mainrail to which it is attached, creating local stress problems. Additionally, for the same transverse force, the load on the panhard rod is about 41% greater than the load experienced on the previously described splayed rods when the latter have an included angle of 90.degree..